it shouldnt make any difference, it has no calories in or anything to upset ketosis. the only problems are that you might drink less water instead and it still contains chemicals that arent really good for you.
I believe they recommend less than 500mls a day -not to say you should have that every day but if you do its not going to do much harm.
also, losses might differ week to week anyway, most people tend to have a good week followed by a smaller loss week, regardless of what they do, but then some people are consistent each week.